This invention relates to remote sensing techniques to detect gas leaks. In particular, mounting a remote sensing instrument on a ground or aerial vehicle that can survey a target area, such as a pipeline, and measuring absorption of upwelling electromagnetic radiation that has passed through gas-filter correlation radiometer (GFCR).
A GFCR is a remote sensing radiometer that uses a sample of the gas as a spectral filter, providing enhanced sensitivity and selectivity to that gas. Incoming radiation is passed through a correlation cell, which is undergoing a gas-density modulation along its optical path. The radiation is then passed through a bandpass filter, which passes only a specific spectral (passband) range selected to cover an absorption band of the gas of interest. The radiation is then measured by an infrared detector. GFCRs have been used in different configurations for over three decades in remote sensing instrumentation.
Methane (CH4) comprises approximately 95% of the composition of natural gas. However, CH4 exists in fairly large quantities in the atmosphere (it is well mixed in the atmosphere with a concentration of approximately 1.7 ppm). Therefore, detecting a gas leak required detection of a small increase on a large background. Events such as passing near a source region of CH4 (such as a farm) or an increase in the altitude of the airplane (an increase in the atmospheric path length) might result in the false signature of a leak.
To reduce the influence of the background, some past attempts have tried to detect the excess CH4 of a natural gas leak by detecting the absorption of CH4 in the infrared wavelength regions where the absorption bands are greatest for example, at 7.8 μm (2180 cm−1) or 3.3 μm (3000 cm−1). This provides the advantage that the upwelling radiation is primarily emitted from the earth's surface. This minimises the background CH4, as only the CH4 located between the remote sensing instrument and the earth's surface is detected. However, for underground pipe—since the temperature of the surface and the leaked CH4 are nearly the same the radiative contrast between the surface and the leaked methane is very small, greatly reducing the detectivity/detectability of the leak. Also, the thermal noise introduced within the instrument itself becomes a serious design constraint.
As the background of CH4 becomes very large, the solar radiation reaching the instrument would have passed through entire atmosphere. The best known satellite instrument to attempt to measure lower atmospheric trace gases using GFCRs was the MOPITT (Measurements Of Pollution In The Troposphere) instrument launched on NASA's Terra satellite. MOPITT was a satellite instrument launched in December 1999. MOPITT was designed to measure the concentrations CH4 in the lower atmosphere utilising the 2.3 μm wavelength. The 2.3 μm CH4 channels of MOPITT failed as the signal-to-noise ratio (SNR) of the measurements did not provide enough resolution to measure the concentration of CH4 to a resolution ≤1%, which was required for global atmospheric chemistry models. As a consequence of this failure, attempts to measure CH4 in lower atmosphere using the 2.3 μm wavelength have been discounted.